Generally methods of forming multi-layer paint films where the object which is being painted is an automobile body are carried out by forming an electrostatic paint film on the object which is being painted and heating and curing and then forming a multi-layer paint film comprising a mid-coat paint film, a base paint film and a clear paint film. Furthermore, at the present time aqueous paints are being used for the mid-coat paint and base paint in order to cut down volatile organic solvents (VOC).
Thus in recent years the so-called three-coat-one-bake (3C1B) system multi-layer paint film forming method in which a base paint film and a clear paint film are formed over a mid-coat paint film which has been subjected to preliminary drying without heating and curing the mid-coat paint film after coating the mid-coat paint on an electrodeposited paint film and forming a mid-coat pant film and these three paint film layers are all heated and cured at the same time has come into use from the viewpoint of energy conservation.
Moreover, a multi-layer paint film forming method with a novel 3C1B system in which an aqueous first base paint, an aqueous second base paint and a clear paint are coated without using a mid-coat paint in this 3C1B type multi-layer paint film forming method has been proposed. In this method of forming a multi-layer paint film the use of the mid-coat base in the coating process is not essential and, since coating can be achieved with just a top-coat booth, it is possible to achieve energy savings.
The use of an aqueous first base paint which includes as an essential component a hydroxyl group-containing non-ionic resin which has a specified acid value and hydroxyl group value which has a specified content of ethylene oxide units and/or propylene oxide units in a multi-layer paint film forming method of the 3C1B type in which an aqueous first base paint, an aqueous second base paint and a clear paint are coated over an electrodeposited paint film has been disclosed in Patent Document 1 as such a method of forming a multi-layer paint film with a 3C1B system in which no mid-coat paint is used.
Furthermore, the use of a first aqueous colored paint which includes titanium oxide which has a specified specific surface area in a multi-layer paint film forming method of the 3C1B type in which a first aqueous colored paint, a second aqueous colored paint and a clear paint are coated over an electrodeposited paint film has been disclosed in an earlier application, Patent Document 2.
An excellent paint film appearance can be obtained in a multi-layer paint film forming method of the 3C1B type in which no mid-coat paint is used and, moreover, a multi-layer paint film forming method with which an excellent paint film appearance can be obtained even in those cases where the aqueous second base paint is coated without carrying out preliminary drying after coating the aqueous first base paint has been obtained by means of the inventions described in Patent Documents 1 and 2.